Track circuits are used in railway systems to detect the presence or absence of a train on rail tracks. Generally, track circuits work by applying power to each rail and a relay coil across the rails. The track itself is separated into defined sections, separated by insulating joints. In order to prevent one circuit from accidentally powering another in the event of insulation failure, the polarity of each section alternates from section to section. When no train is present on the rails, the relay is energized by a track signal current flowing from the power source to the rails. When a train is present on the rails, the axles and wheels of the train shunt the track, so the track signal current to the relay is shorted as well.
Because direct current (DC) track circuits can cause large traction return currents on the rails that overwhelm much smaller track signal currents, alternating current (AC) track circuits are often utilized, which use an AC frequencies in the range of about 91 Hz to 250 Hz. The relays are then designed to detect a specific frequency and to disregard all other AC and DC traction frequency signals. When the traction return current passes through only one rail, the configuration is known as an AC single rail track circuit.
AC double rail track circuits are those in which the traction return current passes through both rails of the track circuit. Electrified railways with AC double rail track circuits utilize impedance bonds as a means for providing an electric connection items that need to be connected for electrification but must stay isolated from track frequencies in order for the track circuit to function. The impedance bond provides a connection between the isolated rails, permitting the traction return current to continue to travel from one section of insulated rail to the next section, while blocking the track signal current from passing outside its relay coil.
Prior art impedance bonds often include a single bond self-contained within a heavy duty cast iron or steel housing, often referred to as a tub, which, with difficulty, could be mounted between the rails, such as the one disclosed in U.S. Pat. No. 4,509,024 to Wilson. Two such bonds are required at each insulating joint. Enclosed within the housing is a core, formed of a band of silicon steel and formed into a U-shape. On each of the two legs of the core are identical copper coils of equal resistance, wound on an identical mandrel to result in identical electrical characteristics. Attached to the end of each coil is a terminal strap that extends outward from the housing.
The U-shaped cores are commonly known in the art as C-cores. The use of C-cores is generally well-known. Wound from silicon steel strips, C-cores are especially suited in transformers where the primary and secondary windings are physically separated.
Prior art impedance bonds made by Power Engineering Industries have a low profile and are designed for installation above tie level and between rails. These units allow for AC and DC propulsion configuration, 300 Amp/Rail AC to 3000 Amp/Rail DC models, and fixed impedance or custom-tunable designs, and are designed to Association of American Railroads (AAR) standards.
Oftentimes, however, despite efforts to balance the two coils in the impedance bond, imbalances still occur on the core. Additionally, high currents often result in overheating of the impedance bonds. The prior art also generally requires several hours in order to install, resulting in delays in scheduled trains that utilize the railway tracks. Another concern is the desire to create low profile units that can be installed between railway line sleepers without protruding into track bed material. C-core impedance bonds generally do not lend themselves to a low profile bond. Furthermore, impedance bonds must be operationally immune from the effects of false signaling that often occurs as a result of high transient currents.
Moreover, terminal straps connecting to the coils in conventional impedance bonds are less robust that ideal and often have numerous bends and excessive vertical profiles that are inconsistent with a low profile impedance bond.
Consequently, there exists a need in the industry for an impedance bond that overcomes the drawbacks of the prior art.